In brief: Because this individuality is not one's own to begin with, once it has been well tamed, well trained, well educated in the discipline of the Aristocrats, it is like a medicine for the world. In and of itself it heals (or moves towards good health) whatever it comes into contact with in this world. So not only is the act of suicide an act of murder in the sense that one is killing an individual that is not one's own, but one is killing one who is of great benefit to the world.

[SN 5 54 9] Vesali Sutta: The Buddha teaches the sutta on meditation on the foul and numerous monks commit suicide. A very early description of the breathing meditation is introduced.

Vesali: A a blending of the Samyutta Nikaya and Vinaya versions of the sutta describing the "Meditation on the Foul", a mass suicide of Bhikkhus, the formulation of the rule concerning killing a human being, and a very early description of the "Breathing" meditation technique.

For a glimpse into the thinking of a Bhikkhu of our time who committed suicide, see Nanavira, Letters, VI(45-48) [Links temporarily unavailable]

The two Patimokkha rules that look to apply in this case are:
1. [Norman/Pruitt:] The Parajika rule (Entailing excomunication):
The rule about a human being:
3. Whatever bhikkhu should intentionally deprive a human being of life, or seek a [life-]taking weapon for him, or should utter praise of death, or should urge him towards death [saying], "Good man, what use to you is this miserable life? Death is better for you than life." having such thoughts in mind and such intentions in mind, in many ways should utter praise of death, or should urge him towards death, he too becomes defeated, not in communion.
and
7. The section on matters entailing [Norman/Pruitt: simple expiation][Bhk. Thanissaro: confession],
g. The section about [Norman/Pruitt: living beings][Bhk. Thanissaro: animals]
61. The rule about intentionally.
[Bhk Thanissaro:] Should any bhikkhu knowingly deprive an animal of life, it is to be confessed.
[Norman/Pruitt:] If any bhikkhu should intentionally deprive a living being of life, there is an offence entailing expiation.

The issue for the observer here (anyone not Bhikkhu Nanavira himself) needs to be confined to the mechanics of the rules of the order as they relate to kamma. One needs to carefully avoid judgment as to the fate of Bhikkhu Nanavira in that unless one is empowered with the Diva Eye this is a matter which cannot be known in that it could have been determined at the very last second by a change of heart and cannot therefore be determined by such logic as "He broke this and that rule and that involves rebirth in hell which is something that would not happen to a Sotapatti, therefore he cannot have been a Sotapatti."
Bhikkhu Nanavira seems to have believed he only committed the lesser offense involving simple expiation. This does not fit with the idea that this body is not one's own and that consequently depriving it of life is depriving a human being of life.
However, mistaken about this or not, there is no specific statement made in the Suttas that I am aware of that would allow one to suggest that breaking a parajika rule would also entail such kamma as would determine a rebirth in Hell or as an Animal...that is to say that would determine that one was not at least a Streamwinner. This is not the same case as the cases that are found where there are those who have broken the Parajika rules who have been reported to have been reborn in Hell  there is (again, as far as my recollection goes) no general statement about such a case.